The present invention relates to a manufacturing process and straightening jig for a hollow tapered rod which is to serve, for example, as a fishing rod or a golf club shaft, more specifically to a manufacturing process which can harden a prepreg-sheet wound around a tapered mandrel without bending deformation and to a straightening jig to be used for perfect hardening of the prepreg-sheet.
The conventional process of manufacturing a hollow tapered rod is as follows. A woven cloth is obtained using at least one of glass and carbon fibers, or a unidirectionally oriented sheet is obtained using a bundle of at least one of glass and carbon fibers. A fiber-reinforced sheet is formed by impregnating a single aforementioned cloth or sheet or a superimposed plurality thereof with a solution of at least one thermosetting resin such as epoxy, polyester or phenol, which are non-hardenable at room temperature, yielding a sheet impregnated with semi-dry resin, i.e., a so-called "prepreg-sheet". The prepreg-sheet is cut to an appropriate size and wound around a tapered iron or alloy mandrel. The wound pre-preg sheet is wrapped with a spiral winding of a cellophane tape or a polyester tape and then placed in an air heating furnace, where it is cured. Finally, after the mandrel is pulled out of the hardened sheet, the wound tape covering the sheet surface is stripped off, yielding a hollow tapered rod.
In the above-mentioned conventional manufacturing process, however, the finished hollow tapered rod is liable to develop an arcuate, convoluted or spiral bend or some combination thereof. These bending deformations are supposedly caused as follows:
(1) The number of layers of windings of prepreg-sheets on the tapered mandrel should be uniform, but in the practical process of mass production it is very difficult to have the start and end of the layers in perfect alignment; in some cases, part of the rod has one more or one less layer than specified because the spiral winding is, respectively, too long or too short. Thus, an arcuate or convoluted bend appears due to local variation in the contracting rate of the reinforcing fibers. PA1 (2) Since the fibers in the prepreg-sheet run spirally to the mandrel, the fibers contract spirally, resulting in a spiral bending of the sheet. PA1 (3) A spiral tape externally covers the prepreg-sheet. Accordingly, with a force acting spirally on the sheet surface, a spiral bending of the sheet develops. PA1 (4)For winding a prepreg-sheet around the mandrel, the start of the sheet cut to an appropriate shape is placed on the mandrel and by applying a hot iron, the resin in the start of the sheet is adhered to the mandrel and thereafter the sheet is wound around the mandrel in several layers. Thereby the phenomenon in paragraph (3) occurs most frequently in the outermost layer of sheet. Meanwhile, with the start of the innermost layer adhered to the mandrel, a difference develops in the degree and direction of tension in reinforcing fibers between the outer and inner layers, thereby causing a compound bending of the sheet. PA1 (5) When the resin is heated and becomes temporarily fluid, a natural balancing takes place between the reinforcing fibers, but this balancing is confined to the intermediate layers which are freer than the outer or inner layers with the result that a general balance cannot be attained. PA1 (6) When carbon fibers are used for reinforcement, the bending of the sheet is aggravated because the tensile strength of carbon fibers is far greater than that of glass fibers.
For these reasons, the bending tendency is the greater, the smaller the diameter of the forward or near-forward section in a multi-sectioned fishing rod. Rods of carbon fibers are found superior as fishing rods or golf club shafts, but their mass production is impossible on account of considerable deformations liable to happen in their manufacture, resulting in their high prices.
Moreover, when the diameter of the mandrel is small, a deformation of the wound sheet causes a deformation of the mandrel, making it impossible to pull out the mandrel.